op-program: Hint reader recovery

Attempt to recover the hint read stream when the NextHint callback fails

op-program/preimage/hints.go

@@ -55,6 +55,8 @@ func (hr *HintReader) NextHint(router func(hint string) error) error {
 		}
 	}
 	if err := router(string(payload)); err != nil {
+		// stream recovery
+		_, _ = hr.r.Read([]byte{0})
 		return fmt.Errorf("failed to handle hint: %w", err)
 	}
 	if _, err := hr.r.Read([]byte{0}); err != nil {

op-program/preimage/hints_test.go

@@ -3,6 +3,7 @@ package preimage
 import (
 	"bytes"
 	"crypto/rand"
+	"errors"
 	"io"
 	"testing"
 
@@ -71,4 +72,21 @@ func TestHints(t *testing.T) {
 		err := hr.NextHint(func(hint string) error { return nil })
 		require.ErrorIs(t, err, io.ErrUnexpectedEOF)
 	})
+	t.Run("cb error", func(t *testing.T) {
+		var buf bytes.Buffer
+		hw := NewHintWriter(&buf)
+		hw.Hint(rawHint("one"))
+		hw.Hint(rawHint("two"))
+		hr := NewHintReader(&buf)
+		cbErr := errors.New("fail")
+		err := hr.NextHint(func(hint string) error { return cbErr })
+		require.ErrorIs(t, err, cbErr)
+		var readHint string
+		err = hr.NextHint(func(hint string) error {
+			readHint = hint
+			return nil
+		})
+		require.NoError(t, err)
+		require.Equal(t, readHint, "two")
+	})
 }